1. Field of the Invention
The present invention relates to a fluorescent rare earth complex showing a high luminous intensity and having a long service life. The invention also relates to a light-emitting medium, a security medium and a lighting device using that complex.
2. Background Art
In recent years, light emitting elements have been improved significantly in luminous intensity and service life, and the market thereof is developing in a wide range around the use for lighting.
Light-emitting diode elements (hereinafter, referred to as “LED elements”) using inorganic fluorescent substances, which are mainly used at present, are becoming remarkably improved in emission efficiency, and hence it is said that white LED elements, particularly, will surpass fluorescent lamps in emission efficiency in the future. However, when LED elements are used in lighting devices, the devices are often used in applications requiring that the elements be excellent in color rendering property as well as in emission efficiency. In spite of that, at present, LED elements using only inorganic fluorescent substances cannot satisfy all the required properties.
It is an already known idea to use organic fluorescent substances in LED elements. Actually, however, LED elements using organic fluorescent substances as luminous bodies are not yet practically used for lighting because of the following reasons.
1) Particularly in the case where organic fluorescent substances are employed in a LED element composed of R, G and B luminous bodies and a near ultraviolet LED light source (which is currently becoming mainly used), the fluorescent organic compounds deteriorate seriously by ultraviolet rays. This is because organic compounds are generally poor in durability against ultraviolet rays. Especially if the compound has an n−π* transition absorption band in the near ultraviolet region, the deterioration proceeds rapidly.
2) Organic fluorescent substances often vary their fluorescence spectra according to the concentration, and hence it is difficult to control the spectra. Further, the intensity of the fluorescence also depends upon the concentration, and concentration quenching often occurs in a high concentration range.
3) The fluorescence spectra are also varied according to the kinds of polymers dispersing the organic fluorescent substances.
As compared with the normal organic fluorescent substances, fluorescent substances of rare earth complexes generally have the following advantages.
1) The luminescence wavelength is essentially ascribed to specific characteristics of the rare earth, and hence is not affected by the dye concentration and the kinds of dispersing polymers. Accordingly, the fluorescent substance exhibits a stable fluorescence spectrum.
2) Although the complex has ligands derived from organic compounds, the organic ligands excited by absorbing light are soon deactivated to the ground state because of energy transfer to the center element. Accordingly, the opportunity for causing irreversible chemical reactions in the excited state is reduced enough to expect satisfying durability against ultraviolet rays.
However, in order to develop the general lighting market, it is required further to improve the luminous intensity and the service life.
As a property greatly affecting the durability, stability of ligands to photochemical reactions can be mentioned. The fluorescent substances irradiated with light of LED are exposed to severe conditions such as strong light and heat, and accordingly the radical (oxidative) deterioration is liable to proceed. If the ligand undergoes a chemical reaction, its coordinating ability is lowered to result in elimination of the ligand and, consequently, the fluorescence intensity is often weakened or the denatured ligand may cause deactivation.
On the other hand, in order to obtain a high luminous intensity, the fluorescent complex is required to be solved well in resin. If the solubility is so poor that the fluorescent substance in the form of particles remains in the resin, emitted light is scattered to lower the luminous intensity. For the purpose of improving the solubility, the structure of the fluorescent substance is studied. For example, JP-A-2003-81986 (KOKAI) discloses a complex comprising phosphine oxide ligands in which all the hydrogen atoms of phenyl groups are replaced with fluorine atoms. However, as far as the present inventors know, there is room for improvement in view of the emission intensity although the disclosed complex is improved in solubility.